EEPROM emulation

[tuxedo0801]

On can read on the official Arduino Zero page (https://www.arduino.cc/en/Main/ArduinoBoardZero):

Memory
The SAMD21 has 256 KB Flash Memory. It also has 32 KB of SRAM and up to 16 KB of EEPROM by emulation.

But there is no EEPROM emulation yet, or?!

The ESP8266 guys also have no EEPROM but flash, and implemented an EEPROM like API:

https://github.com/esp8266/Arduino/blob/master/doc/libraries.md#eeprom

How about doing the same thing for the SAMD?!

[q2dg]

Well, there is https://github.com/cmaglie/FlashStorage ...

[tuxedo0801]

That's not very practical compared to what a usual eeprom-lib gives you w.r.t. usability.

[cmaglie]

The ESP8266 guys also have no EEPROM but flash, and implemented an EEPROM like API:

that's not a drop-in replacement for the EEPROM library, there are two fundamental difference:

• you must call EEPROM.commit() to actually save the changes to flash. This is needed to minimize the amount of writes to the flash memory.
• you must declare how much flash memory you want to use with EEPROM.begin(size). This is needed to allocate a RAM "buffer" to keep the pending changes waiting to be "committed".

That's not very practical compared to what a usual eeprom-lib gives you w.r.t. usability

it depends, which use case you have in mind?

[tuxedo0801]

you must call EEPROM.commit()
you must declare how much flash memory you want to use with EEPROM.begin(size)

that's acceptable from my point of view

it depends, which use case you have in mind?

use case is: use the flash like an eeprom, with almost the same API.

[cmaglie]

use case is: use the flash like an eeprom, with almost the same API.

I could write a wrapper around FlashStorage to emulate the EEPROM API in a similar way to ESP, but what I want to know from you is why you think it's better? What are you trying to do?

The fact that it's almost the same API means that sketch written for the real EEPROM library cannot be used as is, at least you should carefully manage the .commit() action to not consume the Flash lifetime.

[tuxedo0801]

I talked to a lot of not so experienced arduino developers. They all have no problem with the well known EEPROM API. But as soon as it's getting more complex than this, the refuse to use it. And they are all confused by the term " EEPROM by emulation", because Arduino Zero platform does not deliver anything that fits this. And adding an external eeprom just to use an API that is eeprom-like is not ideal, especially because of the already existing (flash) memory the samd already provides.

Also there are already existing libraries that makes use of the EEPROM lib. Porting them to samd with a completely different "storage" api is of course possible, but from my point of view it makes no sense that each and every developer has to create it's own "eeprom api adapter" that is able to deal with the EEPROM api and something completely different on Arduino Zero.

Adding those "begin" and "commit" function with a "#ifdef ..." is not a big thing and keeps the difference between an atmega arduino and the samd arduino at minimum.

From my point of view, the API like the ESP8266 guys created is the best achievable way to adapt "non-eeprom-hardware" to the well known eeprom api. All you have to know/add is those two functions, wrapped by an "#ifdef". Very simple, even for an arduino noob.

[spiderkeys]

I've read the samd21 datasheet from beginning to end and have lived in the weeds of each subsystem for the past year writing custom drivers that are wrapped to emulate the Arduino API to keep them arduino developer friendly. At this point, I agree with @cmaglie that the flash storage interface cannot be used as a drop in replacement for the EEPROM and would suggest that it not even have an official wrapper that emulates the EEPROM API. While it might not be immediately accessible for the less experienced developers you talked to, I would argue that the most prudent route forward would be to expose the storage interface using its own library accompanied by documentation that explains how the flash storage subsystem of these ARM chips works. It ultimately doesn'the serve anyone's interests to implement a crippled or potentially life-shortening interface to the storage simply to keep people from reading a couple paragraphs on how to modify their code to use it pragmatically. If you want such a wrapper library, it is definitely possible, but there are several downsides, as @cmaglie points out.

[q2dg]

I suppose leveraging up an ARM emulated-EPPROM library as an "official" library, side by side the AVR's one is a fool idea, isn't? I say it because maybe in the future (who knows the future?) there will be more Genuino boards based in ARM than in AVR...Nowadays the result is AVR 3 - ARM 2

[tuxedo0801]

I had again a look at this issue and the code by @cmaglie ...
I came up with the idea to simply have a 1k byte[] in a struct and save this to flash with the recommended lib.

pro: use the flash like an eeprom
con: "waste" 1k of RAM

With help of that, it's wrote wrapper (which uses cmaglie's FlashStorage code) that is very similar to the well EEPROM.h API.

There are three new methods:

init() --> call on startup to initially read "eeprom data" from flash
commit() --> write the "eeprom data" to flash: Do this after "bulk eeprom updates" only to keep write-cycles to flash as low as possible
isValid() --> returns true, if "eeprom data" has been written at least once, otherwise will return false (f.i. just after sketch upload, when flash is empty)

By using #ifdef one can call this new methods on samd21 systems only ...

I've not created a project/repository so far. So here's the quick'n'dirty code (feel free to use it or ignore it):

Samd21Eeprom.h

/*
  Copyright (c) 2015 Arduino LLC.  All right reserved.
  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.
  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  See the GNU Lesser General Public License for more details.
  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA


  EEPROM like API that uses Arduino Zero's flash memory. 
  Requirements: cmaglie's Flash Storage: https://github.com/cmaglie/FlashStorage
  (c) 2016, A. Christian
*/

#ifndef EEPROM_h
#define EEPROM_h

#include <FlashStorage.h>

#ifndef EEPROM_EMULATION_SIZE
#define EEPROM_EMULATION_SIZE 1024
#endif

typedef struct {
  byte data[EEPROM_EMULATION_SIZE];
  boolean valid;  
} EEPROM_EMULATION;


class EEPROMClass {

  public:
    EEPROMClass(void);
    uint8_t read(int);
    void write(int, uint8_t);
    void update(int, uint8_t);
    bool isValid();
    void init();
    void commit();

  protected:
    EEPROM_EMULATION _eeprom;
    bool _dirty;
    FlashStorageClass<EEPROM_EMULATION> *_flash;

};

extern EEPROMClass EEPROM;

#endif

Samd21Eeprom.cpp

/*
  Copyright (c) 2015 Arduino LLC.  All right reserved.
  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.
  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  See the GNU Lesser General Public License for more details.
  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA


  EEPROM like API that uses Arduino Zero's flash memory. 
  Requirements: cmaglie's Flash Storage: https://github.com/cmaglie/FlashStorage
  (c) 2016, A. Christian
*/

#include "Arduino.h"
#include "Samd21Eeprom.h"

FlashStorage(my_flash_ptr, EEPROM_EMULATION); 

EEPROMClass::EEPROMClass(void) {
  _dirty = false;
  _eeprom.valid = false;  


  _flash = &my_flash_ptr;
}

uint8_t EEPROMClass::read(int address)
{
  return _eeprom.data[address]; 
}

void EEPROMClass::write(int address, uint8_t value)
{
  _dirty = true;
  _eeprom.data[address] = value; 
}

void EEPROMClass::update(int address, uint8_t value)
{
  _eeprom.data[address] = value; 
}

void EEPROMClass::init()
{
  _eeprom = _flash->read();
}

bool EEPROMClass::isValid()
{
  return _eeprom.valid;
}

void EEPROMClass::commit()
{
  if (_dirty) {
    _eeprom.valid=true;
    _flash->write(_eeprom);
  }
}

EEPROMClass EEPROM;

Issue is now solved to me.

[q2dg]

Well,I wouldn't close this until this piece of code (of similar) gets merged into official EEPROM library repository (https://github.com/arduino/Arduino/tree/8385aedc642d6cd76b50ac5167307121007e5045/hardware/arduino/avr/libraries/EEPROM, which should be moved then to another folder architecture's agnostic). I think some official solution should be given

[tuxedo0801]

Sorry, I don't get it... First, you say that creating such an eeprom like api is a fool idea, and now you want to have it merged into official EEPROM lib repo?

[q2dg]

I'm not from Arduino crew. I only expressed a personal opinion

El 29 sept. 2016 17:44, "Alex" notifications@github.com escribió:

Sorry, I don't get it... First, you say that creating such an eeprom like
api is a fool idea, and now you want to have it merged into official EEPROM
lib repo?

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